This invention relates to a carbon filament, graphite filament, process for production thereof and various forms of the filaments. More particularly, the present invention is concerned with a carbon filament produced by the gaseous phase method, which is easily convertible to a graphite structure, a graphite filament produced by heating the carbon filament and mass forms thereof and fabricated pieces obtained therefrom. The present invention is also concerned with the use of these products.
Carbon filaments are materials which have, in recent years, been aggressively developed as the material for various kinds of composite materials due to their excellent mechanical properties. Conventionally, carbon filaments have been produced by carbonizing organic filaments. In the recent years, it has been attempted to produce carbon filaments by the gaseous phase method in which pyrolysis of hydrocarbons is utilized to form carbon filaments. The carbon filaments produced by the gaseous phase method are excellent in crystallinity and orientality as compared with the carbon filaments produced by the conventional methods. For this reason, the carbon filaments produced by the gaseous phase method not only have a high mechanical strength and high modulus but also are excellent in electrical conductivity and heat conductivity and, therefore, are expected to be useful for electrode materials, heater elements, filters, carriers for catalyst, diaphragms to be used for audio products, reinforcers for plastics, rubber, metals, carbon and ceramics, and the like.
With respect to the carbon filaments used for the conventional filament composite materials, it is known that the smaller the diameter of carbon filaments, the more excellent the reinforcing effect or integrity thereof. Such tendency is attributed to the fact that the smaller the diameter of carbon filaments, the larger the surface of carbon filaments contacting a resin or binder. The wettability of carbon filaments by a resin is not very good. Therefore, it is desirable that the diameter of carbon filaments be as small as possible. However, the carbon filaments which have been produced by the conventional methods in which acrylic fibers are calcined or pitch is made infusible have a diameter of at least about 6 to 10 .mu.m because of difficulties in spinning the precursor filaments.
Carbon filaments produced by pyrolysis of hydrocarbon compounds are described in Japanese Patent Application Laid-Open Specification No. 47-20418/1972 and the method of pyrolysis of hydrocarbon compounds is described in Japanese Patent Application Publication No. 41-12091/1966. However, the carbon filaments disclosed in these references comprise polycrystalline graphites having a laminate structure in which pipe-like graphites are closely mixed with helical graphites and they are present around a fine columnar graphite as an axis, which structure is different from the growth ring structure. Further, the carbon filaments disclosed in the above references have a diameter of several .mu.m to several hundred .mu.m, which diameter is not small enough. Moreover, in the above references, there is not such a description that the carbon filaments have crimps. Still further, the inventions of the above references do not use the catalyst, such as metals, in producing carbon filaments.
U.S. Pat. No. 2,796,331 discloses that carbon filaments can be obtained by pyrolyzing a gas mixture of 20 to 80% methane and 80 to 20% hydrogen gas at1,150.degree. to 1,450.degree. C. for 0.4 to 15 seconds and that hydrogen sulfide is added in a proportion of 0.3 to 4% as a catalyst. However, in this reference as well, metals and the like are not used as a catalyst.
U.S. Pat. No. 3,378,345 discloses that a hydrocarbon gas is pyrolyzed at 700.degree. to 1,200.degree. C. in the presence of a hydrogen gas mixture containing carbon dioxide or water at a concentration of 2.5.times.10.sup.-3 to 0.1% by weight to grow a nucleus and then the nucleus is further grown at a temperature of 1,200.degree. C. or more to form graphite whiskers. The process of this reference is not one in which metals and the like are used as a catalyst.
Japanese Patent Application Laid-Open Specification No. 52-103528 discloses a method that (i) particles of a transition metal, an oxide thereof or a carbide thereof are scattered over a heat resistant substrate plate, (ii) the substrate plate is placed in an electric furnace and the temperature is raised to a level of 1,030.degree. to 1,300.degree. C. and (iii) a stream of a gas mixture of hydrocarbon compounds and hydrogen is supplied into the electric furnace to effect pyrolysis, thus forming carbon filaments on the substrate plate. In this method, particles of a transition metal and the like serve as the nucleus of growth. Such particles are often widely varied in diameter and it is difficult to scatter the particles over the surface of the substrate plate uniformly. Therefore, the carbon filaments produced by this method generally have a diameter of as large as 10 .mu.m or more and the diameter is widely varied. Even if fine carbon filaments are obtained, the lengths of the carbon filaments are short and the carbon filaments have no crimp and difficult to be collected. Further, even if the carbon filaments are subjected to surface oxidation treatment which is intended to improve the adhesiveness between the carbon filament and a resin, the surface of the carbon filaments is difficult to oxidize. Still further, when the carbon filaments produced by this method are used for a metal composite material, the metal composite material in which the carbon filaments produced by this method are used is liable to release the gas inside the metal composite material because the carbon filaments have large hollow portions inside the same. The carbon filaments produced by this method have good graphitizability as compared with PAN type carbon filaments and pitch type carbon filaments. However, the carbon filaments at issue are composed of a large number of kinds of carbons and it is difficult to convert them into graphite filaments.
Japanese Patent Application Laid-Open Specification No. 58-180615/1983 discloses a method in which ultra fine powders of a metal having a high melting point, for example a metal which does not sublimate at 950.degree. to 1,300.degree. C., an oxide thereof, nitride thereof, salt thereof or the like are suspended in a zone for pyrolyzing hydrocarbon compounds to grow carbon filaments. However, the carbon filaments obtained by this method have many branched portions because when the ultra fine powder adheres to a carbon filament in the course of growth, a branched carbon filament grows from the portion to which the ultra fine powder adhered. Further, the diameter of the carbon filaments is large and the carbon filaments have a large hollow portion and no crimp. Still further, not only the activity of the catalyst is low but also the catalyst cannot be sufficiently suspended in a zone for pyrolyzing hydrocarbon compounds because previously produced ultra fine powders as the catalyst are dispersed in a volatile solvent and then used. Therefore, in this method, the yield is insufficient and carbon filaments having a small diameter cannot be obtained efficiently.
Moreover, a process for preparing carbon filaments in a gaseous phase method in which a mixture of a gaseous organic metal compound and a carrier gas or a mixture of a gaseous carbon compound, a gaseous organic metal compound and a carrier gas is reacted has been proposed in European Patent Application Publication No. 136 497 with a view to obviating the above-mentioned drawbacks of the conventional processes. However, this process as well does not ensure efficient production of desirable carbon filaments in high reproducibility.
As is apparent from the foregoing, there have not been obtained desirable carbon filaments which have a small enough diameter and only a few branched portions are uniform in diameter and, therefore, have a sufficient reinforcing effect. Further, there have not been produced desirable carbon filaments of short length for reinforcement which have not only such a high graphitization degree that the carbon filaments are advantageously used as an electrically conductive material, but also a ratio of filament length to filament diameter sufficient to exhibit a reinforcing effect.